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From:   Allen <allen.lkml@gmail.com>
Date:   Mon, 19 Oct 2020 18:51:39 +0530
Message-ID: <CAOMdWS+F=cCK=Rgy-0xk4_mqUFMn1PQBWR8u3JwqTP2AVifxsA@mail.gmail.com>
Subject: Re: [RFC] PCI: allow sysfs file owner to read the config space with CAP_SYS_RAWIO
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     linux-pci@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
        ast@kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Allen Pais <apais@linux.microsoft.com>,
        Allen Pais <allen.pais@lkml.com>
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> > > >
> > > >  Access to pci config space is explictly checked with CAP_SYS_ADMIN
> > > > in order to read configuration space past the frist 64B.
> > > >
> > > >  Since the path is only for reading, could we use CAP_SYS_RAWIO?
> > >
> > > Why?  What needs this reduced capability?
> >
> > Thanks for the review.
> >
> > We need read access to /sys/bus/pci/devices/,  We need write access to config,
> > remove, rescan & enable files under the device directory for each PCIe
> > functions & the downstream PCIe port.
> >
> > We need r/w access to sysfs to unbind and rebind the root complex.
>
> That didn't answer my question at all.

Sorry about that, breaking it down:

When the machine first boots, the VFIO device bindings under /dev/vfio
are not present.

root@localhost:/tmp# ls -l /dev/vfio/
total 0
crw-rw-rw-. 1 root root 10, 196 Jan  5 01:47 vfio

We have an agent which needs to run the following commands (We get
access denied here and need permissions to do this).
echo -n xxxx yyyy > /sys/module/vfio_pci/drivers/pci:vfio-pci/new_id
echo -n xxxx yyyy > /sys/module/vfio_pci/drivers/pci:vfio-pci/new_id

And we want to avoid handing CAP_SYS_ADMIN here. Which is why the
thought about CAP_SYS_RAWIO.
>
> Why can't you have the process that wants to do all of the above, have
> admin rights as well?  Doing all of that is _very_ low-level and can
> cause all sorts of horrible things to happen to your machine, and is not
> really "raw io" in the traditional sense at all, right?


If the above approach is going to cause the system to do horrible things,
then I'll drop the idea.

-- 
       - Allen